There are many instances in which it is desirable to form a metallic coating on a base object or substrate. Various techniques have been employed to form such coatings. These include such techniques as electroplating, welding and high voltage sputtering. While all of these techniques are useful in some applications, they cannot be used interchangeably because they each have certain limitations. For example, if a thick coating of metal is required, i.e., a coating greater than 0.25 mm, then electroplating and sputtering are not useful. A metallic coating which is electroplated to a thickness of 0.25 mm or more is porous and therefore weak. A sputtered coating of such a thickness is impractical because it is virtually impossible to maintain the required high voltage levels for a long enough time to deposit such a thick film of metal. A spray welding process is capable of producing a dense and thick film of metal. However, prior art welding processes deposit metal with a non-uniform thickness. Thus, when a prior art spray welding process is used to coat a substrate, it is necessary to apply a coating that is thicker than a required final coating depth and remove excess metal in a machining operation. Removal of welded metal in a machining operation is time consuming and costly. Additionally, welding produces substantial heat in the substrate. Therefore, welding can only be used on substrates which can tolerate the elevated temperatures which are encountered in the welding process.
When thin metallic coatings are needed and electroplating techniques are used, there is a problem associated with disposal of spent plating solutions which contain various toxic substances. These disposal problems are particularly acute when the metallic coatings must be formed of metals such a chromium or nickel.
Given this set of conditions, it has long been considered desirable to be able to produce high density and uniform metallic coatings in a wide range of precisely controlled thicknesses. The attainment of this goal has heretofore remained elusive. It appears that the most promising technique for applying metallic coatings are those which involve spraying of molten metal onto a substrate. However, prior art metal spraying techniques, while promising, still fall short of the mark. We, the inventors, believe that one of the principal factors that has precluded the achievement of this goal is that prior art metal spraying techniques have used spraying conditions that do not optimize liquid droplet size and velocity. Our analysis and insights have led us to believe that if a highly uniform and predictable spray plume of metal can be formed with good repeatability, then a workable coating process would be attainable. We have found that a droplet of sprayed molten metal must be sufficiently large so that its does not lose its internal heat and solidify prior to reaching a substrate. Additionally the droplets must be sufficiently small so that a collection of the droplets will form a non-porous coating on the substrate. We have found that by controlling the velocity of transit of the molten droplets, we are able to achieve a desired balance of droplet size so that a non-porous coating can be produced with a variety of different metals.
In addition to our findings relating to metallic coatings, we have determined that it is possible to generate objects of near-net shape by spraying metal into a mold.
Various prior art devices have been employed to produce liquid sprays. Many of these well known devices employ aspiration methods to produce a spray plume. For example, in U.S. Pat. No. 4,919,853 (Alvarez et al.) issued Apr. 24, 1990 there is an apparatus and method described which employs a uniquely shaped supersonic nozzle to aspirate a liquid into droplets. This system is described as having applicability to spraying of molten metals. However, the system described in the above mentioned patent is not practical for a metal coating operation in which a wide range of coating thicknesses, metal types and substrate types are used. The Alvarez et al. patent teaches a technique for mathematically defining a particular nozzle shape which will produce a desired spray plume at a single flow rate. A single flow rate is, at best, useful for spraying only one type of material under a very limited range of conditions. If a different flow rate is needed to spray a different material or to spray in different conditions, then it is necessary to create another unique nozzle to produce the desired flow rate. Any one construction of the Alvarez et al. device is not sufficiently flexible in its application to be useful over any but the narrowest range of metal types and coating conditions.
The Alvarez et al. apparatus relies on the principle of aspiration for its operation. Aspiration in a nozzle occurs only within a very narrow range of gas flow conditions. In the context of a manufacturing operation, it is impractical to replace a nozzle in a spraying apparatus whenever it becomes necessary to change the type or density of a metallic coating being applied to a substrate or object.
It is desirable therefore to provide a system that will produce a metallic spray that is comprised of uniformly sized droplets. It is additionally desirable that such a system be capable of producing such uniformly sized droplets in a wide range of sizes and flow rates so that the system can be used in a wide range of applications.